Liquid could be mixed by machinery and propeller, perpetuated by the liquid itself through pumped circulation, and agitated by other means. Gas mixing has the advantages of scouring action and flotation effect. The scouring action can be used to cleanse a surface such as a wall surface of hollow fiber membrane and a reactor. The flotation effect can contribute to the uniform suspension of solid particles in liquid without precipitation.
Gas mixing may be continuous, or intermittent or pulsed. Continuous gas mixing is simple, generally achieved using an aeration device such as aeration diffusers for providing a continuous gas supply. Mixing requires water turbulence. Intermittent gas agitation can obtain the water turbulence with lower energy by their discrete bubble stirring but continuous gas agitation requires higher energy consumption for same purpose. In many cases, the agitated liquid requires a period of time such as a few seconds to become calm; non-oriented continuous gas supply instead causes the offset of the energy. Intermittent gas supply can be realized by valve switches, but frequent switching particularly the frequency within the range of a few seconds, will cause fatigue failure of the valve. Pulsed gas agitator being made by using of the principle of the abrupt change of air pressure has no mechanical change; the gas supply could be continuous, but the gas discharge could be pulsed.
The immersed ultrafiltration/microfiltration hollow fiber membrane has been widely used in liquid-solid separation process, such as membrane bioreactor (MBR) used in the sewage treatment process, and membrane chemical reactor (MCR) used in the water treatment and recycling process. In these processes, air scouring is crucial in order to prevent the film surface from being contaminated. However, since air scouring requires higher energy consumption, how to save energy and achieve a predetermined air scouring effect has become a key technology. Gas wash can swing the membrane fibers, so that the contaminants cannot be accumulated on the membrane surface. At the same time, the gas makes the solids such as sludge or flocculant to be evenly distributed in the liquid.
Although mechanical agitation is effective in the liquid-liquid and liquid-solid mixing processes, they are sometimes subject to the limitations of the medium, for example, strong acids and high salinity of the liquid have a corrosive effect on metal. Using an inert gas as the stirring power can overcome such insufficiencies.